With reference to FIG. 1, a common arrangement of an SPM instrumentation 10 is shown, by taking a scanning tunneling microscope (STM) setup as a non-limitative example. A probe P, associated with a specimen S to be imaged, comprises a scanning element, such as a sharp tip T or cantilever, usually made of tungsten or platinum-iridium, carried at the end of a piezo-electric support member M. The piezo-electric member allows for performing both distance regulation between the tip and surface and scanning of the specimen under investigation. Specifically, by advancing or retracting the tip with respect to the specimen surface (along the z-axis, according to the common orientation) one can keep constant the distance between the atoms on the tip and the atoms in the surface, and control the acquisition of data in a non-contact mode. The piezo-electric member is controlled so as to move the tip in the directions parallel to the surface of the specimen (commonly identified by x- and y-axes) for achieving the scanning of the exposed surface or modifying the topography of the specimen by precisely manipulating the surface atoms. Usually, a combination of displacements of the tip in the x-, y- and z-directions is obtainable by suitable geometries of the piezo-electric member, and the most common one is a tubular geometry as depicted, having four external quadrant electrodes E whose length can be changed by means of the piezo-electric effect induced by electric fields in the piezo-electric member. Acquisition and control sections include electronic circuit arrangements performing reading of signals representative of the interactions between tip and specimen, as well as feedback control for continuously adjusting the position of the tip and define its trajectory over the surface of the specimen, respectively. The acquisition section includes amplifying means A for conditioning the electronic signal output from the tip and indicative of the physical (reading) parameter under investigation (e.g. the tunneling current in a STM arrangement or the small attractive/repulsive force between the tip and the surface with which the tip interacts in a AFM arrangement), and a processing unit U is arranged for measuring the resulting value of said parameter and for displaying information relating to it in an intelligible view onto a screen D. The control section is arranged downstream the processing unit and comprises a feedback loop from the processing unit to a driving arrangement C of the piezo-electric member, adapted to output control voltages to the electrodes so as to actuate the tip according to the operating mode of the microscope.
The time resolution of commercially available SPM instruments is one of their most severe limitations since the typical image acquisition times are of the order of seconds or more. However, most of these dedicated SPMs do not have enough time resolution to allow for observing manipulation events and local, non equilibrium dynamics on the time scale of some milliseconds, i.e. imaging rates of video rate and beyond. As a consequence, fundamental details of atomic scale dynamical processes occurring at surfaces, which are often of great fundamental and technological interest, are partially or totally lost: surface diffusion, phase transitions, self-assembly phenomena, film growth and etching, chemical reactions, conformational changes of molecules are only some examples among the infinite variety of dynamics occurring at surfaces. Motivated by this interest, driving scanning probe microscopes (SPM) at imaging rates close to video rate and beyond has become one of the core technical challenges of surface science in the last two decades.
The main factors that limit the scan speed in scanning probe microscopes are the limited bandwidth of the components of the electronic control systems, namely the control and signal detection electronics, and the mechanical resonances of the scanner. A critical limit to the time resolution is given by mechanical instabilities of the SPMs that appear as soon as the scanning frequencies reach or exceed the intrinsic resonance frequencies of the imaging system.
The fastest SPM technique is Scanning Tunneling Microscopy and in this field most of the FastSPM development has taken place; in the following we refer mainly to this technique by way of non-limitative example. The main restriction to the scan speed imposed by STM electronic control systems is the bandwidth of the fundamental components, namely the preamplifier for detection of the tip-specimen interaction (i.e. the tunneling current in STMs) and the feedback system, which has to respond or account for the spatial variations of the detected signal. The required bandwidth is in the tens of kHz range concerning the probe motion and in the MHz range concerning the preamplifier. Moreover, data acquisition and real-time processing speed can also limit the achievable performance: for instance, the acquisition of a 100×100 pixels2 image at an imaging rate of 100 Hz with 16 bits requires the acquisition system to handle a minimum data stream of about 2 Mb/s (data transmission, elaboration and visualization).
While suitable, high bandwidth and high power operational amplifiers can be readily found on the market, high bandwidth preamplifiers with low noise characteristics in the 109V/A (108V/A) amplification range and a frequency-independent response are not commercially available above the 50 kHz (200 kHz) bandwidth. This problem has been addressed with custom-built preamplifiers with higher bandwidths, up to 600 kHz in the 109V/A range. This approach has the obvious drawback of requiring a custom design of the preamplifier, whereas in our experience, with standard 200 kHz cutoff and suitable signal post-processing, useful signal to noise ratio can be achieved also at much higher frequencies (up to 800 kHz).
When imaging in constant current mode, the limited response time of the feedback system is also of concern. Different workarounds have been developed, consisting for instance in: i) a “hybrid” feedback loop between constant current and constant height mode; ii) a combination of a dual feedback system with a dual piezo actuators, where the latter are driven by the fast and the slow feedback loops separately; iii) a combination of a feedforward and a feedback control where the height information from the last scanned line is used to predict the following line in such a way that the feedback only compensates for the differences between the two lines. The latter method is of great interest because it can correct for the unavoidable specimen tilting, which is often the major effort for the feedback loop.
The most serious limitation to the scan speed, however, is imposed by the mechanical resonance frequencies of the scanner assembly. Resonances are always excited when driving the tip close to an eigenmode frequency, and also when inducing the displacement with lower frequency signals that contain high frequency harmonics, e.g. a triangular wave.
When the scanner is driven close to resonance frequencies, two major problems arise: i) the amplitude of the probe movements in the direction of the stimulus (i.e. the amplitude of the tip oscillation or scan area) cannot be a priori predicted anymore, and ii) a coupling to other directions (orthogonal to the surface or in the slow scan direction) can occur, resulting in tip crashes and/or distorted imaging. Since the piezoelectric actuators, as coupled to their support, have by default a complicated frequency spectrum, lateral resonances often couple to vertical resonances (orthogonal to the surface), with strong impact on the obtainable imaging resolution, a crucial parameter in STM.
The problem of exciting unwanted resonances in the displacement of the tip driven by signals containing high frequency harmonics has been successfully solved with different methods, the simplest one consisting in using a pure sinusoidal wave as tip driving waveform, combined to image post-processing in order to correct for the non-uniform tip speed. The problem of piezo resonances in the direction orthogonal to the surface (that impose a limit to the scan speed) has been tackled by developing SPM scanners with very high eigenfrequencies or by using suitable active damping/vibration compensation schemes, which, however, only partially limit the effects of such resonances. An alternative but inspiring approach consists in using the resonances of the probe (in this case a tuning fork, i.e. Atomic Force Microscopy instead of STM), rather than avoiding them, thus exploiting the steady state resonant oscillation to drive either the probe itself or small specimens at high frequency. This approach, albeit still involving the development of custom built systems, proves the feasibility of scanning with resonating probes. In other words, not all the mechanical resonances harm the resolution of the STM, but a method is missing that determines unequivocally which resonances are harmful and have to be avoided.
In order to overcome these problems, the previous research has focused almost exclusively in tailoring both the electronics and the mechanics of the SPM systems with increased stiffness or lower mass, with the aim of increasing the available bandwidth and pushing the resonance frequencies of the scanners above the desired scan speeds. This has brought to life a variety of highly specialized, custom-made SPM scanners and control systems, developed to reach high imaging speeds. Different solutions have been implemented, from conical piezo geometries with increased stiffness as a substitute for the traditional cylindrical geometry, flexure-based scanners with reduced crosstalk between lateral and vertical scan directions, up to recent micromechanical systems (MEMS) composed of spring actuated membranes with extremely low mass.
However, the design, testing and commissioning process for these instruments involves a substantial effort in terms of time and money investment, not to speak about the highly specialized technical know-how that is required. The complexity of this approach is clearly unbearable for most research labs interested in fast SPM measurements. On the contrary, little or no attention has been paid to the possibility of driving commercially available, non-tailored SPMs at high imaging speed, owing to the lack of knowledge on how to deal with the presence of resonant modes of the scanner within the range of the scan frequencies of interest.
The most important information needed in order to assess the speed capabilities of an SPM instrument is the frequency response of the scanner. Various approaches have been devised to measure this function, where the difference between the methods stems from the choice of the sensor for the piezo displacement. Conceptually speaking, the simplest solution is to use a capacitive or inductive displacement sensor, but this method is not so appealing since it requires an invasive, non-trivial modification of the scanner head. In a very simple, but powerful approach, piezo displacements have recently been measured by simply exciting the motion through one of the electrode quadrants and using the voltage induced by the displacement in the opposite quadrant as a probe. Last but not least, in the case of STMs, the tunneling current can act as an extremely sensitive, simple and effective, z-displacement sensor.
These methods have been applied up to date almost exclusively for the characterization of custom-built SPMs.